1. Field of the Invention
The invention is directed to a high-frequency-weldable polymer mixture and to sheets or molded articles containing this polymer mixture.
2. Description of the Related Art
It is known that plastics parts can be welded in the high-frequency field. In this case, thermoplastic parts in the form of sheets or molded articles to be connected are heated to the required welding temperature in the capacitor field of a high-frequency voltage source without an auxiliary die. The sheets are seam-welded (overlapping seams welded so as to withstand shearing stress or peeling stress). High-frequency-weldable materials are, above all, thermoplastics with a polar makeup, i.e., plastics in which are built in molecules, atoms or side groups which cause a charge transfer in the molecule (dipoles). When a thermoplastic of this kind enters a high-frequency alternating field, the polar groupings carry out oscillations corresponding to the excitation frequency. The oscillations lead to heat development in the plastic. The dielectric loss factor tan .delta. is generally used as a characteristic value for the transformation of electric field energy into heat. As a rule, the sheets are not connected along the entire surface. Welding is generally restricted to line-type or frame-type contours and heating is restricted to the respective welding web region.
However, because of their nonpolar character, polyolefins cannot be processed in this way. In order to be able to weld polyolefins in the high-frequency field also, U.S. Pat. No. 3,336,173 proposes mixing in polyamides with the polyolefins. However, molded articles made from such materials have the disadvantage that the obtained weld seams do not have adequate strength or that tearing occurs next to the weld seam.
EP-A-0 406 568 describes a polypropylene-polyamide molded compound which, in addition to a polypropylene homopolymer and/or a polypropylene copolymer and a polyamide, contains an olefinic unsaturated carboxylic acid and/or an olefinic unsaturated carboxylic acid derivative. EP-A-0 406 568 contains no reference to the high-frequency weldability of the described molded compound.
DE-A-41 42 271 describes molded articles which contain essentially polyolefins, polyamides and adhesion promoters or bonding agents. The bonding agent is preferably a copolymer of styrene and ethylene butylene blocks grafted with maleic anhydride.
The known propylene-polyamide compounds have the disadvantage that they have a low dielectric loss factor at room temperature, as a result of which they can only absorb small amounts of energy in the high-frequency alternating field. Therefore, a disadvantageously high energy input and high welding power are required for welding this material. On the other hand, the dielectric loss factor increases sharply as the heating of the material caused by the absorption of energy in the high-frequency field increases, resulting in a significant drop in the breakdown voltage or dielectric strength and an increased risk of dielectric breakdown. This leads to an increased reject rate.
Further, it is known that copolymers of ethylene and vinyl acetate are high-frequency-weldable. For example, JP-A-06287362 discloses a high-frequency weldable mixture of polyolefins with ethylene vinyl acetate copolymers (EVA). However, molded articles made from mixtures of this kind have only a limited thermal dimensional stability. Sheets made from such mixtures with a high EVA content tend to stick together at elevated temperatures.